Duplex metal article



April 9, 1935.

R. H. BROWN 1,997,166

DUPLEX METAL ARTICLE Filed Oct. 20, 1953 IN VENT OR I fiobcr/H Bray/7.

Patented Apr. 9, 1935 UNITED STATES PATE DUPLEX METAL ARTICLE Robert H. Brown, New Kensing'ton,- Pa., assignor to Aluminum Company of America, Pittsburgh,

Pa., a corporation of Pennsylvania Application October 20, 1933, Serial No. 6943446 4 Claims.

This invention relates to duplex metal articles, particularly of the type comprising a base of aluminous metal (aluminum or aluminum base alloy) provided on one or more of its surfaces 5 with an aluminous metal coating resistant to corrosion.

To obtain a high degree of corrosion resistance in aluminum alloy articles without sacrifice of ,the structural or other properties which recommend the particular alloy for its intended use, the alloys are coated on one or more-surfaces with a layer of aluminum or an alloy of aluminum more resistant to corrosion than the base metal. Coatings of high purity aluminum have found wide application for these purposes, as such material has a very high inherent corrosion resistance.. However,.it has been found that even high purity aluminum, after extended exposure to corrosive conditions, is sometimes subject to a localized corrosion which penetrates through the coating to the base metal, forming pits in the surface of the article which are relatively deep and of small circumference. This pitting de; stroys the surface appearance of thevduplex metal article as well as impairing the p'rotection-afford-' ed the base metal by the coating. 'Furthermore, such duplex metals possess the disadvantage that at any point where the core material is exposed in contact with the coating metal, as for example along a cut edge of the material or where the coating hasbeen perforated, an electrolytic type of corrosion is set up which may cause a preferential or selective attack of the base metal with a consequent impairment of one or another of its structural properties; for example, strength, elongation or yield point. This preferential or selective attack onthe core metal canoe cur when the core metal has an electrode potential greater than that of the coating metal, whereby voltaic action may be set up between the two metals in contact with the corroding medium, thus causing a flow of current which tends to dissolve the core metal. This type of corrosive action has prevented the use of aluminum and certain of its alloys having relatively 'high electrode potentials as a core or'base metal for duplex aluminous metal articles, since it has been impossible to provide them with a coating of higher electrode potential which possessed an inherent corrosion resistance comparable with that of a pure aluminum coating.

It is an object of the present invention to producean improved aluminous duplex metal article consisting of an aluminum or aluminum alloy base and a coating of an alloy consistingsubstan tially of aluminum having an inherent resistance to pitting by corrosion greater than that of high purity aluminum. Another object of this invention is to produce an improved aluminous duplex metal having an aluminum alloy coating of high inherent resistance to pitting by corrosion and of high electrode potential, and in which aluminum and most aluminum alloys may be used as a base metal, whereby any desired set of struc-. tural properties may be obtained in the'duplex metal article.

I have discovered that certain aluminum alloys containing zinc and magnesium are possessed of an extraordinary resistance to pitting 'by corrosion, as well as high electrode potentials,

and that when such alloys are .used as coatings improved aluminous duplex metals may be produced using an aluminous metal core of substan-- tially any desired structural properties. As disclosed in my copending U. S. application, Serial No. 694,444, filed October 20, 1933, I have found that zinc, when added to aluminum, has the property of markedly increasing the electrode poten-' tial of the aluminum. I have now foundthat a further addition of magnesium to such aluminum-zinc alloys imparts to the alloy a property of remarkable resistance to pitting by corrosion which is greater even than that of aluminum of high purity. In forming these alloys, the zincmay be added in amounts up to about 5.0 per cent, but I have generally found that 0.2 to 3.0 per cent is effective for producing any required electrode potential. The magnesium is preferably added in amounts of 0.5 to 6.0 per cent. In addition to their high resistance to pitting by corrosion and their high electrode potentials, these aluminumzine-magnesium-alloy coatings have a property of hardness which is also very advantageous for certain purposes.

The choice of the proper composition of the aluminum-zinc-magnesium alloy to be used as the coating material in combination with a base of any given structural aluminum alloy depends upon a predetermined knowledge of the electrode potential of the structural alloy in question. The coating alloy containing the magnesium and zinc should have an electrode potential higher than that of the base 'metal. These different electrode potentials are readily measured, in accordance with methods long known in the art, against a standard electrode. Throughout this specification and claims, when I say that the electrode p0 tential of the coating metal should be higher than that of the base metal, I mean that the coating metal should be moreelectron'egative with respect to the standard electrode than is the base metal; While no; fixed potential diflerence is required between the electrode potential of the base metal and that of the coating metal, and.

substantial protection may be given any base metal by a coating metal of higher electrode potential, it is generally desirable that there be a relatively wide difference in the electrode potential of the two metals; since I have found that with greater differences in potential closed and the potential there usually occurs less attack on the base metal.

A convenient standard electrode for measuring the potentials above mentioned may be readily selected from those now in regular use for similar measurements. For the purposes of this description of the invention, the calomel electrode has been selected as a standard. In measuring the electrode potential in question, the aluminum alloy to be tested is made one element of an electrolytic cell; the calomel electrode is made the other element. The electrolyte is a normal solution of sodium chloride containing 0.3 per cent by weight of hydrogen peroxide. The circuit is difference of the elements is measured on a potentiometer. The calomel electrode used is'made up in the usual way of mercury in contact with mercurous chloride, and in contact with this a normal potassium chloride solution saturated with mercurous chloride. I

The base or core metal, that is to say the layer or body of metal from which the duplex metal article of my invention usually derives its principal structural properties, may be any aluminum alloy having the properties which it is desired to obtain in the finished article. If, for example, it is desired that the article have high strength and hardness, an alloy of the duralumin type may be used as the base, such as alloys A, B, or 0 below mentioned. Or if it is desired to obtain an article which combines the properties of extreme ductile ity and workability, alloys of the type of alloys D, E, and F below mentioned may be used. Also, alloys possessing known combinations of'properties may be employed, such as any of the following:

Alloy CompoeitionPereentage by weight-Balance aluminum mxoeozzrm bwnmuowiv In forming the duplex metal article of my invention, any of the known methods of forming such duplex metal articles may be used. One

method which I have found particularly satisfactory isto cast the base alloy against the coatlng alloy as described in the patent of E. H. Dix, No. 1,865,089, issued June 28, 1932. The article may then, if desired, be mechanically worked, as for example by rolling or drawing or other suitable operation, to form the particular type of duplex article desired, such as sheet, rod, or other form of material. The working with intermediate heating incident to such forming operations.

tends to improve the bond between the coatin and the metal. Duplex material may also be produced by spraying or otherwise .piacing the coating alloy on the base metal. This method may be used to advantage where irregular shapes, such as castings, are to be coated.

rosion greater than my invention. A coating coating metal of an aluminum-magnesium-zinc alloy containing 0.15 per cent zinc and 0.2 per cent magnesium of the same thickness were exposed to an accelerated corrosion test. In this test the samples were subjected to corrosive action of sawdust wet with a mixture of acetic acid and sodium chloride. The sample of high purity aluminum coating metal was pitted through in 5 days. On the other hand-the sample of aluminum-zinc-magnesium alloy coating metal, while it showed some corrosion, had not been penetrated by pitting after 21 days.

The duplex metal articles of my invention may be provided on one or all exposed surfaces with a protective coating alloy, as, shown in Figs. 1 and 2 of the drawings. Fig. 1 shows a duplex article comprising a base I of an aluminum structural alloy provided on its top and bottom surfaces with an aluminum-magnesium-zinc alloy coating 2 and 2. In Fig. 2 is shown incross section a duplex article in tubular form comprising a base 3- of an aluminum structural alloy provided on its inner surface with an aluminum-magnesium-zinc alloy coating 4. This type of material is used, for example, in the manufacture of pipes used to conduct material corrosive to the structural alloy.

1 claim: I

1. A duplex metal article comprising an aluminous metal base aluminum alloy containing magnesium and zinc having an inherent resistance to pitting by corrosion greater than that of high purity aluminum and having an electrode potential higher than that of the base metal as determined by reference to a standard electrode.

2. A duplex metal article consisting-of a base of aluminous metal provided with a coating of an alloy consisting predominantly of aluminum and containing 0.2 to 3.0 per cent zinc and 0.5 to 6.0 per cent magnesium, the alloy of the coating having an inherent resistance to pitting by cornum and having an electrode potential higher than that of the base metal as determined by reference to a standard electrode.

3. A duplex metal article comprising an aluminous metal base provided with a coating of an aluminum alloy containing magnesium and zinc having an inherent resistance to pitting by corrosion greater than that of high purity aluminum and having an electrode than that of high purity aluminum and higher than that of the base metal asdetermined by reference to a standard electrode.

4. A duplex metal article comprising an aluminous metal base having an electrode potential at least equal to that of substantially pure aluminum provided with a coating of an aluminum alloy containing magnesium and 0 having an inherent resistance to pitting by er than that of high purity aluminum and having an electrode potential higher than that of the base metal as determined by reference to a standard electrode.

' ROBERT H. BROWN.

for example, are

provided with a coating of an that of high purity aiumipotential higher orrosion great- 

